It is known that metal substrates can be joined with an anaerobic adhesive composition. Anaerobic adhesive compositions are well known for their ability to remain in a liquid, unpolymerized state in the presence of oxygen and to cure to a solid state in the absence of oxygen. To bond substrates with an anaerobic adhesive composition, the composition is coated on the surfaces to be joined, and the surfaces are brought tightly together to remove air (and oxygen) from around the composition to induce polymerization of the composition and formation of a definitive bond.
Anaerobic adhesive compositions, also referred to as “anaerobic resins,” are widely used in the field of mechanics to bond substrates and to join various mechanical parts, such as wheels, pulleys, or ball bearings on their axles. As such, anaerobic adhesive compositions are used to replace traditional bonding techniques such as press assembly, banding, and brazing, and to eliminate the need for supplementary joining pieces such as keys, pins, washers and locking screws. Anaerobic adhesive compositions also have an important role as sealing and gasketing adhesives, and to retain and tighten fastening systems such as screws, nuts, bolts, pins, and the like.
Anaerobic adhesive compositions are generally based on polymerizable polyacrylic ester or acrylate monomers, such as alkylene glycol diacrylate, which are capable of radical polymerization at room temperature.
The compositions also contain a cure system that improves the speed and/or the bond strength of the composition. Various cure systems have been developed, primarily focusing on efficiently performing the redox reaction, which is the basis for anaerobic chemistry. Cure systems generally include organic peroxides or hydroperoxides as an activator of the polymerization reaction. Such peroxide compounds are known to serve as a catalyst to generate free radicals, which initiate free radical curing of the polymerizable anaerobic adhesive monomers.
To increase the speed at which peroxide free radicals are generated, and to speed up the completion of the cure, an acidic reducing agent is commonly used as an accelerator for the peroxide. Typical accelerators include amines and imides, such as tertiary amines, polyamines, cyclicamines and arylamines. Various other nitrogen compounds, including hydrazine derivatives such as ethyl carbazate, N-aminorhodanine, acetylphenylhydrazine, para-nitrophenylhydrazine, and para-tolysulfonylhydrazide, have also been suggested to increase the setting speed on ferrous substrates, but at the price of poor storage stability. It is also difficult to achieve polymerization of an anaerobic adhesive composition on nonferrous surfaces or treated surfaces, such as galvanized or bichromated steel, with most hydrazine derivatives. To further improve the cure speed, various combinations of compounds, in various relative amounts, have also been explored as accelerators. Identifying an optimal cure system from such combinations is, however, a complex and unpredictable process.
Further, to join substrates made of “slow” or relatively inactive materials such as stainless steel, zinc, dichromate, and cadmium, the substrate may have to be coated with a separate primer coating before applying the adhesive composition. Using a primer coating, however, is disadvantageous, since it requires an additional step that is inconvenient and costly. It also requires a solvent that can be environmentally harmful, which would require special handling and disposal.
Thus, it would be desirable to provide an anaerobic adhesive composition with improved curing and bonding properties and that is simple to manufacture and use. It would be further desirable to provide an anaerobic adhesive composition that can be used to join a wide variety of substrate materials without requiring a separate primer coating or other special treatments.